The animal whose absence would have the greatest negative effect on the ecosystem is the wolf.
Wolves are apex predators that help regulate the populations of both deer and foxes. Without wolves, the populations of deer and foxes would likely increase significantly, which could lead to overgrazing or overbrowsing of vegetation by deer and potentially disrupt the balance of the ecosystem. Additionally, without wolves, foxes would have no natural predators, which could further disrupt the population dynamics of rabbits and other species in the ecosystem. Therefore, the absence of wolves would create the most significant imbalance.